Gradient residual stress and fatigue life prediction of induction hardened carbon steel S38C axles; Experiment and simulation

Qin, T. Y.*; Hu, F. F.*; Xu, P. G.   ; Zhang, H.*; Zhou, L.*; Ao, N.*; Su, Y. H.  ; Shobu, Takahisa  ; Wu, S. C.*

Gradient distribution of triaxial residual stresses to a depth of several millimeters is retained in middle carbon steel S38C axles after high-frequency induction hardening, which has become a critical concern for fatigue structural integrity. To address this, the axial, hoop, and radial gradient residual strains inside the axles were measured for the first time by advanced neutron diffraction. The SIGINI Fortran subroutine was then adopted to reconstruct the global initial residual stress field from the measured data. Experimental and simulation results show that residual stresses of about -520 MPa (axial), -710 MPa (hoop), and -40 MPa (radial) residual stress were retained below the axle surface. Subsequently, the fatigue crack propagation behavior of S38C axles was numerically investigated in the framework of fracture mechanics. The calculated results clearly show that the compressive residual stresses at a depth of 0?3 mm from the axle surface lead to a low crack growth driving force, and that fatigue cracks do not propagate as long as the crack depth is less than 3.7 mm for hollow S38C axles. These results further indicate that the maximum defect size allowed in routine inspections is acceptable from a safety and economic point of view. Accurate measurement and characterization of the global gradient residual stress field through experiments and simulations can provide an important reference for optimizing the mileage intervals of nondestructive testing (NDT) of surface defects in these surface-strengthened railway axles.